1. Technical Field
The invention relates to an apparatus and method of monitoring the average temperature in energized electrical conductors such as power lines so as to assure safe clearance from the ground. More particularly, the invention relates to an apparatus and method for measuring the temperature and sag of energized electrical conductors such as power lines in real time as they change with varying electrical load on the line as well as varying environmental conditions causing thermal expansion where the temperature monitor assists in the determination of the maximum power transmission feasible through such conductor while maintaining safe clearance from the ground.
2. Background Information
With deregulation of utilities including electrical utilities, it is now more important than ever that utilities be efficient in the delivery of services since competition now exists. In addition, deregulation has opened up new markets for individual utilities and as a result all utilities are seeking to expand while defending their home region. As a result, there is an ever-increasing need for electric utilities to transfer more power through their existing power lines, that is to maximize transmission through existing resources.
One hurdle to increased power transmission is clearance between the power line and the ground or structure. Government regulatory codes can mandate safety considerations for overhead or suspended power lines that require utilities to provide adequate clearances between the ground and/or structures under the power line. This clearance may need to be maintained at all times including in all weather conditions and under all actual load conditions.
Thus, clearance can be is one of the considerations to electrical utilities because power lines sag under increasing power loads and as a result limitations are placed on the ampacity or maximum load a line is allowed to carry. The reason for this is that power lines sag as load is placed on the power line and that sag increases as the load increases. This sag-load correlation is the result of heat causing the temperature of the conductor to rise and further causing thermal expansion of the conductor corresponding to load levels. Heat is generated in the conductor by the resistance losses resulting as electrical current flows through it. This heat causes thermal expansion of the conductor. As load increases more heat is generated resulting in ever increasing thermal expansion of the power line causing the power line to sag closer to the ground. Because government regulations mandate the minimum clearance, utilities must assure that this minimum clearance is never violated.
In addition, numerous other factors also affect the suspended power line and the sag therein including ambient temperature (warmer temperatures increase sag), and wind speed and direction (wind usually cools the line and thus decreases sag). All of these factors, and primarily the thermal expansion, are considerations to electrical utilities as indicated above because steps must be taken to assure that adequate clearances as required by law are maintained. As a result, ampacity, or the maximum load is generally limited to less than maximum levels as a safety factor to assure that minimum clearance is maintained at all times and under all weather and load conditions. It is often typical that safety is a significant factor and thus maximum load is significantly affected.
Adequate clearance regulations are necessary because power lines, after being installed in relation to the ground or structures, may later sag so as to become too close to the ground or structures resulting in significant safety concerns. One such concern is that when power lines sag too close to the ground, electrical shock or contact with the lines becomes more feasible and thus safety is at issue. Another such concern is that electric flashover scenarios are possible as lines become too close to electrically grounding objects such as the ground or structures, and such electric flashover can result in extensive damage.
During installation and before a load is placed on the lines, the power lines can be installed such that sufficient clearance is achieved. This can readily be done by mere visual sight alignment or by simple measurement techniques measuring the distance from the lowest part of the line to the ground or nearest structure. It is even possible to very roughly account for factors such as ambient temperature, wind speed, wind direction and other environmental factors using conservative assumptions and historical knowledge. It is noteworthy though that such conservative assumptions result in significantly less than maximum line loading.
However, once an electrical load is placed on the power lines, various load factors cause the power lines to sag. One of these factors is thermal expansion of the power line under load as mentioned above, and specifically under a continuously varying load. The clearance between a suspended electrical conductor and the ground decreases as the conductor sags due to this thermal expansion under load. Thermal expansion can be correlated to load in the conductor such that increased load results in increased thermal expansion. Due to the desire to transmit as much power as possible through electrical conductors, it is important to monitor this thermal expansion and the resulting sag.
Full utilization of transmission lines requires analysis of sag and clearance with respect to these sag factors and the thermal expansion factor. In theory, this allows for the calculation of maximum load that still provides for minimum clearance as required by safety regulations. Current technology is such that several approximate methods provide for such approximation or calculation.
One method for determining power line sag involves measuring the temperature of the conductor at a particular spot on the power line. Mathematical modeling is then used to calculate the sag. This method is an approximation because the conductor temperature varies based upon location radially within the conductor, location on the line, wind, exposure to elements, etc. and thus the approximation may be inaccurate.
Safety factors are instituted to assure minimum clearances at all times thereby not optimizing the thermal expansion and sag allowed. Some of the problems of this method are due to its approximating qualities rather than accurate calculations. Other disadvantages and/or problems result from the inability to measure the temperature at all points, instead of sample points. As a result of these and other disadvantages and problems, additional safety factors may need to be added to assure minimum clearances, but as a result optimization suffers.
Alternatively, the environmental factors have been measured on the spot and then used to calculate the actual conductor temperature in conjunction with the above mentioned conductor temperature reading. This approach is time consuming, labor intensive, indirect and often subject to large errors.
Monitoring sag in a power line by only monitoring temperature can have disadvantages. These disadvantages may include conservative current ratings resulting from an assumed combination of worst case cooling conditions. Worst case cooling condition can include a combination of highest expected ambient temperature and lowest wind speed, both of which are may not occur under actual conditions. Monitoring of sag using temperature can also include adding a time function to the calculation that is to intermittently calculate rather than worst case scenario. The actual measurement of conductor sag or alternatively the ground clearance can also be measured manually. These measurements may be done with actual measuring, using acoustics, microwaves, and laser beams, although none of these methods may be practical. The equipment can be bulky, heavy and expensive. The equipment is typically installed on the ground under the conductor and thus must be left unattended where it is subject to vandalism, and it reduces the clearance at the center portion of the line where it is installed.
Other methods of measuring power line sag include measuring the power line tension at a suspension point. Because the line tension is affected by its inclination angle, by knowing the tension, the inclination angle can be determined and thus the sag. There are limitations and/or disadvantages associated with this tension measuring method. First, load cells used to measure the tension must be capable of measuring very small changes in a large static tension that is continuous on the line; and as a result, the accuracy of the sag determination is based upon the accuracy of the load cell and its capability of measuring small tension changes. Second, often load cells must be installed in-line which requires de-energizing and cutting of the line; and as a result, significant labor expense and line downtime is incurred. Finally, many of the current tension reading load cells must be installed on the grounded end of insulators holding the line at dead-end structures; and as a result, calculations cannot be performed on all spans.